Control apparatus of vibration-type actuator and control method of vibration-type actuator

ABSTRACT

Provided is a control apparatus of a vibration-type actuator for generating an elliptical motion of contact portions by a common alternating current signal including a frequency determining unit for setting a frequency of the alternating current signal. The frequency determining unit sets the frequency of the alternating current signal for changing an ellipticity of the elliptical motion, within a frequency range such that ellipticity changing frequency ranges set for the vibrators are overlapped, and the ellipticity changing frequency ranges are set for the vibrators as frequency ranges between an upper limit and a lower limit, such that the lower limit is a maximum resonant frequency at a time of changing the ellipticity, and the upper limit is larger than the lower limit and is a maximum frequency for the relative movement of the driving member.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a control apparatus of a vibration-typeactuator and a control method of a vibration-type actuator.

2. Description of the Related Art

Actuators are conventionally proposed which subject a given mass pointon a plate-like vibrator to elliptical motion (driving member) to drivea driving member.

As a basic configuration of a vibration-type actuator with a plate-likevibrator, such a configuration as illustrated in Japanese PatentApplication Laid-Open No. 2004-320846 is known. FIG. 8A is a perspectiveview illustrating an example of the external basic configuration of thevibration-type actuator in Japanese Patent Application Laid-Open No.2004-320846.

As illustrated in FIG. 8A, a vibrator in the vibration-type actuatorincludes an elastic member 4 formed of a metal material shaped like arectangular plate. The elastic member 4 includes a piezoelectric element(electromechanical energy transducer) 5 joined to a back surfacethereof. A plurality of protrusions 6 are provided on a top surface ofthe elastic member 4 at respective predetermined positions.

According to this configuration, applying an AC voltage to thepiezoelectric element 5 allows simultaneous generation of secondarybending vibration in a long side direction of the elastic member 4 andprimary bending vibration in a short side direction of the elasticmember 4. This excites elliptical motion in the protrusions 6.

Then, the driving member 7 is brought into contact with the top portions(contact portions) of the protrusions 6 under pressure and then linearlydriven by elliptical motion of the protrusions 6. That is, theprotrusions 6 act as a drive unit for the vibrator.

FIG. 8B is a schematic diagram illustrating an example of a polarizationarea of the piezoelectric element 5 in the vibration-type actuatorillustrated in FIG. 8A.

Furthermore, FIGS. 9A and 9B are perspective views illustrating avibration mode of the elastic unit 4. FIG. 9C is a diagram illustratingelliptical motion excited in the protrusions 6 of the elastic unit 4.

The piezoelectric element 5 is subjected to a polarization process andincludes two electrodes A1 and A2, as illustrated in FIG. 8B.

AC voltages V1 and V2 in phase with each other are applied to the twoelectrodes A1 and A2, respectively, to excite the rectangular elasticunit 4 into primary bending movement with two nodes extending in adirection parallel to the long side direction. This corresponds to afirst vibration mode illustrated in FIG. 9A.

Furthermore, the AC voltages V1 and V2 out of phase with each other areapplied to the two electrodes A1 and A2, respectively, to excite therectangular elastic unit 4 into secondary bending movement with threenodes extending in a direction parallel to the short side direction.This corresponds to a second vibration mode illustrated in FIG. 9B.

Then, the first vibration mode and the second vibration mode arecombined together to excite elliptical motion in the protrusions 6. Atthis time, when brought into contact with the protrusions 6 underpressure, the driving member can be linearly driven.

Here, the first vibration mode illustrated in FIG. 9A allows activationof an amplitude (hereinafter referred to as a Z-axis amplitude)displaced in a direction perpendicular to the surface of the contactportion (hereinafter referred to as the contact surface brought intocontact with the driving member under pressure in the protrusions 6.

Furthermore, the second vibration mode illustrated in FIG. 9B allows anamplitude (hereinafter an X-axis amplitude) displaced in a directionparallel to the contact surface to be excited in the protrusions 6.

Combination of the first vibration mode and the second vibration modeallows elliptical motion to be excited in a predetermined one of theprotrusions 6 as illustrated in FIG. 9C. The ratio in magnitude betweenthe Z-axis amplitude and the X-axis amplitude is hereinafter referred toas ellipticity of elliptical motion.

FIG. 10A is a graph illustrating the amplitudes in the first vibrationmode and the second vibration mode observed when the difference in phasebetween the two-phase voltages V1 and V2 is changed between −180 degreesand 180 degrees.

When the difference in phase between the two-phase AC voltages V1 and V2applied to the respective two electrodes A1 and A2 of the polarizedpiezoelectric element is changed between −180 degrees and 180 degrees,the amplitudes in the first vibration mode and the second vibration mode(P2) are as illustrated by P1 and P2 in FIG. 10A, respectively.

In FIG. 10A, the axis of abscissas indicates the phase difference. Theaxis of ordinate indicates the amplitudes in the first amplitude modeand in the second amplitude mode.

Combination of the first vibration mode and the second vibration modeallows elliptical motion to be excited in the protrusions 6. Changingthe phase difference between the AC voltages V1 and V2 to be appliedallows adjustment of ellipticity of elliptical motion excited in thepredetermined protrusion 6.

FIG. 10A illustrates, in the lower part thereof, elliptical shapescorresponding to the phase differences on the axis of abscissas. Thedirection of driving by the vibration-type actuator, which provideslinear driving, can be switched by switching between the positive signand negative sign of the phase difference between the AC voltages V1 andV2.

Moreover, the direction and speed of driving can be consecutivelychanged by consecutively changing the phase difference starting with anyvalue, with the sign appropriately changed between the positive one andthe negative one (for example, consecutively changing the phasedifference between 90 degrees and −90 degrees, with the signappropriately changed between the positive one and the negative one).

Concerning the driving speed, the following phenomenon (which is calleda cliff drop phenomenon) occurs as illustrated in FIG. 10B. The drivingspeed peaks at the resonant frequency and decreases slowly on a higherfrequency side of the resonant frequency, while decreasing rapidly on alower frequency side of the resonant frequency.

Furthermore, as is generally known, the speed can be increased bysetting the frequency of the AC voltage applied to the piezoelectricelement closer to the resonant frequency. The speed can be reduced bysetting the frequency of the applied AC voltage further away from theresonant frequency.

As such a vibration-type actuator, an apparatus can be provided whichexerts a driving force increased using a plurality of vibrators.

However, when the vibration-type actuator is configured to drive thedriving member, using a plurality of vibrators, the following problemmay occur.

When a common frequency is applied to each of the plurality of vibratorsin order to simplify the circuit configuration of a control apparatus ofthe vibration-type actuator, the vibration-type actuator operatesunstably if the resonant frequency varies among the vibrators. Thus, thevibration-type actuator needs to drive the object without usingfrequency regions corresponding to the unstable operation.

In view of the above-described problem, an object of the presentinvention is to provide a control apparatus of a vibration-type actuatorand a control method of a vibration-type actuator in which thevibration-type actuator configured to drive the driving member using aplurality of vibrators can drive the object stably even with a variationin resonant frequency among the vibrators.

SUMMARY OF THE INVENTION

The present invention provides a control apparatus and method for avibration-type actuator described as follow.

A control apparatus for a vibration-type actuator, wherein thevibration-type actuator includes a plurality vibrators each having acontact portion contacting an object to be driven, such that, responsiveto a common alternating current signal to the plurality vibrators, theplurality vibrators move, through the contact portions, the objectrelative to the vibrators, to generate an elliptical motion of theobject, wherein the control apparatus comprises a frequency determiningunit for setting a frequency of the alternating current signal, and thefrequency determining unit sets the frequency of the alternating currentsignal for changing an ellipticity of the elliptical motion, within anoverlapping frequency range in which frequency ranges of ellipticitychanging each set for each of the plurality of vibrators are overlapped,and the frequency ranges of changing the ellipticity are set for thevibrators as a frequency ranges between an upper limit and a lowerlimit, such that the lower limit is a maximum resonant frequency in allcase of changing the ellipticity, and the upper limit is larger than thelower limit and is a maximum frequency for the relative movement of thedriving member.

A control method for a vibration-type actuator, wherein thevibration-type actuator includes a plurality vibrators each having acontact portion contacting an object to be driven, such that, responsiveto a common alternating current signal to the plurality vibrators, theplurality vibrators move, through the contact portions, the objectrelative to the vibrators, to generate an elliptical motion of theobject, wherein the control method comprises a step for setting afrequency of the alternating current signal, and, in the step frequencydetermining, the frequency of the alternating current signal is set forchanging an ellipticity of the elliptical motion, within an overlappingfrequency range in which frequency ranges of ellipticity changing eachset for each of the plurality of vibrators are overlapped, and thefrequency ranges of changing the ellipticity are set for the vibratorsas a frequency ranges between an upper limit and a lower limit, suchthat the lower limit is a maximum resonant frequency in all case ofchanging the ellipticity, and the upper limit is larger than the lowerlimit and is a maximum frequency for the relative movement of thedriving member.

Further features of the present invention will become apparent from thefollowing description of exemplary embodiments with reference to theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view illustrating the external configuration ofa vibration-type actuator according to a first exemplary embodiment ofthe present invention.

FIG. 2 is a graph illustrating the relationship between the drivefrequency and driving speed of a vibrator in the vibration-typeactuator.

FIG. 3 is a block diagram illustrating the configuration of a controlapparatus of the vibration-type actuator according to the firstexemplary embodiment of the present invention.

FIG. 4 is a block diagram illustrating an ellipticity determining unitand a frequency determining unit in the vibration-type actuator indetail according to the first exemplary embodiment of the presentinvention.

FIG. 5 is a graph illustrating the relationship between the drivefrequency and driving speed of the vibrator in the vibration-typeactuator according to the first exemplary embodiment of the presentinvention.

FIG. 6 is a flowchart for a vibration-type actuator according to asecond exemplary embodiment of the present invention.

FIG. 7 is a diagram illustrating the configuration of a vibration-typeactuator according to a third exemplary embodiment of the presentinvention.

FIG. 8A is a perspective view illustrating an example of an externalbasic configuration of a vibration-type actuator in a conventionalexample, and FIG. 8B is a schematic diagram illustrating an example of apolarization area of a piezoelectric element in the vibration-typeactuator illustrated in FIG. 8A.

FIGS. 9A and 9B are perspective views illustrating a vibration mode ofan elastic member of the vibration-type actuator in the conventionalexample, and FIG. 9C is a diagram illustrating elliptical motion excitedin protrusions of the elastic member.

FIG. 10A is a graph illustrating amplitudes in a first vibration modeand a second vibration mode, and FIG. 10B is a graph illustrating therelationship between the frequency and speed of the vibrator.

DESCRIPTION OF THE EMBODIMENTS

Preferred embodiments of the present invention will now be described indetail in accordance with the accompanying drawings.

First Exemplary Embodiment

The configuration of a control apparatus of a vibration-type actuatoraccording to a first exemplary embodiment of the present invention willbe described with reference to FIG. 1.

The vibration-type actuator according to the present exemplaryembodiment includes a plurality of vibrators each at least with anelectromechanical energy transducer and an elastic member.

An alternating current of a drive frequency common to theelectromagnetic energy transducers in the respective plurality ofvibrators is applied to the electromagnetic energy transducers to drivea driving member which is in contact with the plurality of vibrators viacontact portions thereof.

FIG. 1 illustrates a vibration-type actuator including two vibratorsconfigured to relatively drive one driving member. The vibration-typeactuator is configured to cause relative movement of a linearlyextending driving member in a longitudinal direction thereof. The twovibrators can generate a double thrust.

Vibrators 8 a and 8 b illustrated in FIG. 1 move integrally owing to aholder (not shown in the drawings), relative to a driving member 3.

As illustrated in FIG. 1, the vibrator 8 a of the vibration-typeactuator includes an elastic member 4 a formed of a metal materialshaped like a rectangular plate. A piezoelectric element(electromechanical energy transducer) 5 a is joined to a back surface ofthe elastic member 4 a.

Two protrusions 6 a are provided on a top surface of the elastic member4 a at respective predetermined positions.

The vibrator illustrated in FIG. 1 subjects the protrusions toelliptical motion so as to drive the driving member which is infrictional contact with upper portions (contact portions) of theprotrusions. Similarly, in the vibrator 8 b, a piezoelectric element 5 bis joined to an elastic member 4 b. Two protrusions 6 b are provided atrespective predetermined positions on the elastic member 4 b.

Each of the piezoelectric elements 5 a and 5 b includes two groups ofelectrodes A1 and A2 illustrated in FIG. 8B. Each of the piezoelectricelements 5 a and 5 b is subjected to a polarization process in thedirection of the sheet of FIG. 8B.

In the vibrator configured as described above, the ellipticity ofelliptical motion excited in the protrusions 6 a and 6 b illustrated inFIG. 1 can be changed by changing the phase difference between two-phaseAC voltages (two-phase alternating current) to be applied.

The vibrator illustrated in FIG. 1 can be increased in speed by makingthe drive frequency of the AC voltage applied to the piezoelectricelement of the vibrator closer to a resonant frequency as illustrated inFIG. 10B.

Furthermore, the speed of the vibrator can be reduced by making thefrequency of the AC voltage applied to the piezoelectric element of thevibrator further away from the resonant frequency.

Additionally, the vibrator is characterized in that the driving speedpeaks at the resonant frequency and gradually decreases on a higherfrequency side of the resonant frequency, while decreasing rapidly on alower frequency side of the resonant frequency.

Furthermore, the movement speed of the driving member can be increasedby increasing an X-axis amplitude of elliptical motion excited in theprotrusions 6 a and 6 b.

In addition, the movement speed of the driving member can be stablyreduced by decreasing the X-axis amplitude, with a Z-axis amplitudemaintained at a predetermined value.

Additionally, the direction of relative movement of the driving membercan be switched by switching the phase difference between the two-phaseAC voltages.

The present exemplary embodiment enables adjustment of ellipticity ofelliptical motion excited in the predetermined protrusions 6 a and 6 bwhen the phase difference between the AC voltages applied to electrodesV1 and V2 of the piezoelectric element 5 illustrated in FIG. 8Bdescribed above is changed between −180 degrees and 180 degrees.

FIG. 10A illustrates, in the lower part thereof, elliptical shapescorresponding to the phase difference on the axis of abscissas.

The driving direction and speed can be consecutively changed byconsecutively changing the phase difference between 90 degrees and −90degrees with the sign appropriately changed between a positive one and anegative one.

The above-described configuration excites vibration in each of thevibrators 8 a and 8 b in FIG. 1 to enable driving of the driving member(slider) 3.

Furthermore, the vibrator according to the present invention is notlimited to the above-described exemplary embodiment. The vibrator maysubject the protrusions to elliptical motion based on a combination ofvibration in the vibration mode in which the Z-axis amplitude is excitedin the protrusions and vibration in the vibration mode in which theX-axis amplitude is excited in the protrusions. Specifically, besidesthe above-described embodiment, an embodiment is possible in which theprotrusions are subjected to elliptical motion based on a combination oflongitudinal stretching vibration of the vibrator and bending vibrationthereof.

Now, the driving characteristics of one vibrator illustrated in FIG. 1will be described.

FIG. 2 is a graph illustrating the relationship between the phasedifference and drive frequency and driving speed of the AC voltagesapplied to the electrode A1 and the electrode A2 (see FIG. 8B) when onevibrator drives one driving member. For example, as illustrated in FIG.2, as the phase difference starts at 90 degrees and approaches 0 degree,the amplitude in the second amplitude mode decreases to reduce thedriving speed. Furthermore, as the phase difference starts at 90 degreesand approaches 0 degree, the resonant frequency increases.

Here, the driving characteristics obtained with the phase differencechanged will be described.

For example, if the drive frequency is set to the value of the resonantfrequency corresponding to a phase difference of 60 degrees, and thephase difference is shifted from 90 degrees toward a smaller phasedifference side, then the cliff drop phenomenon is avoided while thephase difference is between 90 degrees and 60 degrees, because the drivefrequency is higher than the resonant frequency. However, when the phasedifference is shifted toward values smaller than 60 degrees, the cliffdrop phenomenon occurs abruptly. That is, when the drive frequency isset to the value of the resonant frequency obtained when the phasedifference is 60 degrees, and the phase difference is changed, theoperation is unstable when the phase difference is smaller than 60degrees.

However, if the drive frequency is set to the value of the resonantfrequency corresponding to a phase difference of 10 degrees, and thephase difference is shifted from 90 degrees toward a smaller phasedifference side, then the cliff drop phenomenon is avoided while thephase difference is between 90 degrees and 10 degrees, because the drivefrequency is higher than the resonant frequency corresponding to thephase difference of 10 degrees.

That is, if the vibrator is driven with the drive frequency fixed andwith the phase difference changed, the likelihood of the cliff dropphenomenon can be reduced by driving the vibrator with the phasedifference set to correspond to a frequency higher than the resonantfrequency corresponding to a smaller phase difference.

A resonant frequency 24 corresponding to a phase difference of 0 degreeas described above is highest, and is thus defined as a lower limitvalue for control with the phase difference changed.

Now, the driving characteristics obtained with the frequency changedwill be described.

For example, when the drive frequency is swept up toward frequencieshigher than the resonant frequency with the phase difference set to 90degrees, the driving speed decreases gradually. Then, the driving speedlowers abruptly and the driving member stops operation at a portion 23of the drive frequency range. Thus, a frequency higher than the lowerlimit value and at which the driving member stops relative movement isdefined as an upper limit value for phase difference control.Furthermore, the upper limit value may be, instead of the portion 23 atwhich the driving member stops operation during the above-describedsweep-up process, a frequency corresponding to a portion at which thespeed rises rapidly when the drive frequency is swept down from a valuesufficiently higher than the resonant frequency. That is, according tothe present invention, the upper limit value may be set equal to afrequency higher than the lower limit value and at which the drivingmember stops relative movement.

The range of frequencies between the upper limit value and the lowerlimit value described above is set to be an ellipticity changingfrequency range. Changing the ellipticity within the ellipticitychanging frequency range allows avoidance, during driving, of the cliffdrop phenomenon and the portion at which the driving member stopsoperation as a result of a rapid decrease in driving speed.

The configuration of the control apparatus of the vibration-typeactuator according to the present exemplary embodiment will be describedwith reference to a block diagram in FIG. 3.

The control apparatus includes a position instruction generating unit 17configured to generate a target value for the driving member and towhich an operation quantity determining unit 22 is connected via acomparing unit 18 on the output side thereof.

The comparing unit 18 compares the target value output by the positioninstruction generating unit 17 with the current position of the drivingmember which is output by a position detecting unit 16.

The operation quantity determining unit 22 calculates the quantity e ofoperation of the vibration-type actuator based on the comparison resultfrom the comparing unit 18.

The operation quantity determining unit 22 includes a PI controller or aPID controller. The position detecting unit 16 detects the position ofthe driving member 3, and includes, for example, a linear scale or anencoder.

The vibrator 8 a is configured as illustrated in FIG. 1 described above.The vibrator 8 a includes the elastic member 4 a formed of a metalmaterial shaped like a rectangular plate, the piezoelectric element 5 ajoined to the back surface of the elastic member 4 a, and the pluralityof protrusions 6 a provided on the front surface of the elastic member 4a.

Similarly, the vibrator 8 b is configured as illustrated in FIG. 1described above. The vibrator 8 b includes the elastic member 4 b formedof a metal material shaped like a rectangular plate, the piezoelectricelement 5 b joined to the back surface of the elastic member 4 b, andthe plurality of protrusions 6 b provided on the front surface of theelastic member 4 b as a driving portion.

The driving member 3 is illustrated in FIG. 1 and connected to theoutput side of the vibrators 8 a and 8 b.

An ellipticity determining unit 19 and a frequency determining unit 20are connected to the output side of the operation quantity determiningunit 22; the ellipticity determining unit 19 sets a ratio for an ellipsein the above-described elliptical motion, and the frequency determiningunit 20 sets the frequency of an alternating current.

The ellipticity determining unit 19 is configured to be able to set theratio of the X-axis amplitude and Z-axis amplitude in the ellipticalmotion caused in the protrusions of the vibrators 8 a and 8 b based onan output from the operation quantity determining unit 22. Theellipticity determining unit 19 is further configured to be able to seta phase difference allowing the ratio to be achieved.

The drive frequency determining unit 20 is configured to be able to setthe driving frequencies of AC voltages to be applied to the vibrators 8a and 8 b based on the output from the operation quantity determiningunit 22.

Moreover, output sides of the ellipticity determining unit 19 and drivefrequency determining unit 20 are connected to a drive signal generatingunit 21.

The drive signal generating unit 21 generates a two-phase alternatingcurrent with the frequencies determined by the frequency determiningunit 20 and the phase difference determined by the ellipticitydetermining unit 19.

A booster 25 is connected to an output side of the drive signalgenerating unit 21. The booster 25 boosts the two-phase alternatingcurrent generated by the drive signal generating unit 21, and appliesthe boosted two-phase alternating current to the piezoelectric elementsof the vibrators 8 a and 8 b.

The booster 25 may be formed of a power amplifier, a switching element,a DC/DC circuit, or a transformer circuit.

The functions of the ellipticity determining unit 19 and drive frequencydetermining unit 20 according to the present embodiment will bedescribed with reference to FIG. 4.

In each graph in FIG. 4, the axis of abscissas indicates the operationquantity e. The axis of ordinate indicates one of a phase difference θand a drive frequency fr.

The value of the operation quantity e output by the operation quantitydetermining unit 22 is input to the ellipticity determining unit 19.

The ellipticity determining unit 19 uses this input value to determinethe ellipticity based on settings expressed in the graphs.

The operation quantity e output by the operation quantity determiningunit 22 is also input to the drive frequency determining unit 20. Thedrive frequency determining unit 20 then determines the drive frequencyfr based on the settings expressed in the graphs.

As illustrated in the graphs in FIG. 4, when the operation quantity ehas a small absolute value, the phase difference determined by theellipticity determining unit changes. When the operation quantity e hasa large absolute value, the frequency determined by the frequencydetermining unit 20 changes.

Thus, areas with the frequency and the phase difference constant thereinare provided so as to prevent the phase difference from changing whenthe frequency changes, while preventing the frequency from changing whenthe phase difference changes. The frequency fixed when the phasedifference changes are hereinafter referred to as an ellipticitycontrolling frequency (fe).

Now, the ellipticity controlling frequency (fe) set when a plurality ofvibrators with different resonant frequencies drive the driving memberusing a common drive frequency will be described; this aspect is mostimportant in the present invention.

FIG. 5 illustrates the relationship between the drive frequency and thephase difference and the driving speed observed when one of the twovibrators with different resonant frequencies drives one driving member.

FIG. 5 is a graph illustrating the driving speed observed when the phasedifference between the two-phase voltages V1 and V2 applied to thepiezoelectric element illustrated in FIG. 8B is changed.

The drive frequency vs. driving speed observed when the phase differencefor the vibrator 8 a is changed between 90 degrees and 10 degrees is asillustrated at 90 degrees (a) to 10 degrees (a) in FIG. 5.

The resonant frequency corresponding to a phase difference of 0 degreefor the vibrator 8 a is a resonant frequency 24 a corresponding to aphase difference of 0 degree illustrated in FIG. 5. Similarly, the drivefrequency vs. driving speed observed when the phase difference for thevibrator 8 b is changed between 90 degrees and 10 degrees is asillustrated at 90 degrees (b) to 10 degrees (b) in FIG. 5. The resonantfrequency corresponding to a phase difference of 0 degree for thevibrator 8 b is a resonant frequency 24 b corresponding to a phasedifference of 0 degree illustrated in FIG. 5. Furthermore, when thevibrator 8 a places the driving member in relative movement, the maximumfrequency at which the driving member stops relative movement isillustrated at 23 a. When the vibrator 8 b places the driving member inrelative movement, the maximum frequency at which the driving memberstops relative movement is illustrated at 23 b.

In the present embodiment, the ellipticity changing frequency range (a),the range of driving frequencies set by the frequency determining unitbased on the characteristics of the respective plurality of vibrators,is determined as follows.

That is, as described above, the lower limit value of the ellipticitychanging frequency range is defined as the resonant frequencycorresponding to the phase difference of 0 degree (the lower limit valueis defined as the maximum resonant frequency obtained when theellipticity determining unit changes the ellipticity).

Furthermore, the upper limit value is defined as the maximum frequencyat which the driving member can be driven when the driving member isdriven at a frequency higher than the lower limit value.

The ellipticity changing frequency range (a) lies between the thusdetermined lower limit value and upper limit value.

Similarly, the ellipticity changing frequency range (b) is determinedbased on the characteristics of the vibrator 8 b. In the followingdescription, the vibrators 8 a and 8 b drive one driving member.

For example, when the vibrators 8 a and 8 b drive one driving member,the cliff drop phenomenon does not occur even if the phase differencefor the vibrator 8 b is changed at the resonant frequency of thevibrator 8 a corresponding to a phase difference of 30 degrees.

However, the cliff drop phenomenon occurs when the phase difference forthe vibrator 8 a is changed from 90 degrees toward a smaller phasedifference side.

Then, the driving speed of the vibrator 8 a decreases rapidly, thussuppressing the driving speed of the vibrator 8 b.

Furthermore, the driving speed becomes unstable. To avoid this stateduring driving is the object of the present invention as describedabove.

That is, when the driving member is driven at a frequency lower than theellipticity changing frequency range (c), the cliff drop phenomenon mayoccur in any of the plurality of vibrators which has a higher resonantfrequency.

On the contrary, when the driving member is driven at a frequency higherthan the ellipticity changing frequency range (c), the driving speed ofany of the plurality of vibrators which has a lower resonant frequencymay decrease and pass beyond the portion of the frequency range at whichthe vibrator stops operation.

In contrast, according to the present embodiment, the ellipticitydetermining unit is configured to be able to change (control) theellipticity within the overlapping range of driving frequencies betweenthe ellipticity changing frequency ranges set by the frequencydetermining unit based on the characteristics of the respectiveplurality of vibrators.

Specifically, the ellipticity controlling frequency (fe) illustrated inFIG. 4 is set equal to the ellipticity changing frequency range (c),which is the overlapping portion between the ellipticity changingfrequency ranges of the two vibrators.

Thus, the driving speed can be prevented from being unstable duringdriving when one driving member is placed in relative driving and when acommon frequency is applied to a plurality of vibrators.

Second Exemplary Embodiment

A second exemplary embodiment will be described which corresponds to anexample of a configuration for setting the upper limit of theellipticity changing frequency, the highest frequency of a frequencyrange used for driving by a vibration-type actuator, and the lower limitof the ellipticity changing frequency, the lowest frequency of thefrequency range.

The present embodiment provides a control method of a vibration-typeactuator in which a plurality of vibrators drives one driving member. Inthis method, a characteristics detector detects the characteristics ofeach of the plurality of vibrators.

Then, the following are set based on the results of detection of thecharacteristics of each vibrator: the upper limit value of theellipticity changing frequency, the highest frequency of the frequencyrange used by the plurality of vibrators to drive one driving member,and the lower limit value of the ellipticity changing frequency, thelowest frequency of the frequency range.

A control apparatus of the vibration-type actuator in which a pluralityof vibrators drive one driving member has a configuration similar tothat in the first exemplary embodiment. Thus, description of theconfiguration is omitted.

Now, detection of the output characteristics of each vibrator accordingto the present embodiment will be specifically described with referenceto a flowchart in FIG. 6.

First, in STEP 1, the vibrator 8 a of the vibration-type actuator, inwhich the vibrators 8 a and 8 b illustrated in FIG. 1 drive one drivingmember 3, is set in a characteristics detector (not shown in thedrawings).

The characteristics detector (not shown in the drawings) is an apparatusincluding a sensor configured to detect the driving characteristics ofan vibrator in an actuator including the one vibrator and one drivingmember as illustrated in FIG. 8A.

Then, in STEP 2, the frequency to be applied to the vibrator 8 a is setto a sufficiently large value.

Then, in STEP 3, the phase difference for the vibrator 8 a is set to 90degrees.

When the phase difference is changed, the driving speed of thevibration-type actuator is maximized at the set phase difference.

Then, in STEP 4, the frequency and phase difference set in STEP 2 andSTEP 3 are applied to the vibrator 8 a to start driving.

Then, in STEP 5, the speed is detected. The speed detected in this caseis that of the relative driving between the vibrator 8 a and the drivingmember.

Then, in STEP 6, comparison is carried out to determine whether thespeed detected in STEP 5 is equal to or larger than 0. If the detectedspeed is higher than 0, the process proceeds to STEP 8.

Furthermore, if the speed detected in STEP 5 is zero, the detectordetermines that the driving member and the vibrator fail to makerelative movement. The process proceeds to STEP 7.

Then, in STEP 7, the frequency is reduced. The process then proceeds toSTEP 5.

Here, if the driving member and the vibrator fail to make relativemovement, the operation between STEP 5 and STEP 7 is repeated.

Then, in STEP 8, the frequency being applied to the vibrator 8 a isstored in a memory (not shown in the drawings) as the upper limit valueof the ellipticity changing frequency.

Then, in STEP 9, the phase difference applied to the vibrator is set to1 degree.

This is to detect the resonant frequency at as small a phase differenceas possible because the resonant frequency of the vibrator increaseswhen the phase difference is shifted from 90 degrees toward a smallerphase difference side.

Here, the phase difference is set to a sufficiently small value, 1degree.

In subsequent STEP 10, the detector detects whether or not the cliffdrop phenomenon is occurring, in which the driving speed decreasesrapidly. If the cliff drop phenomenon is not occurring, the processproceeds to STEP 11. If the cliff drop phenomenon is occurring, theprocess proceeds to STEP 12.

In STEP 11, the frequency is reduced. If the cliff drop phenomenon isnot detected, the operation in STEP 10 and STEP 11 is repeated.

In STEP 12, the frequency being applied to the vibrator 8 a is stored inthe memory (not shown in the drawings) as the lower limit value of theellipticity changing frequency.

Then, in STEP 13, the vibrator set in the characteristics detector ischanged from the vibrator (a) to the vibrator (b). Thereafter, theprocess proceeds to STEP 2 to repeat the above-described operation. Theoperation allows the output characteristics of the vibrator 8 b to bedetected after detection of the output characteristics of the vibrator 8a.

In the method carried out between STEP 1 and STEP 14, the outputcharacteristics of the vibrator 8 a are detected and then the outputcharacteristics of the vibrator 8 b are detected. This allows the outputcharacteristics of each of the vibrators to be detected as illustratedin FIG. 5.

Then, in STEP 15, an ellipticity changing frequency range is determinedwhich lies between the upper limit value and lower limit value detectedin STEP 1 to STEP 14 when the vibrator 8 a is set in the characteristicsdetector. Furthermore, an ellipticity changing frequency range isdetermined which lies between the upper limit value and lower limitvalue detected when the vibrator 8 b is set in the characteristicsdetector. Then, the overlapping range (common range) between the twoellipticity changing frequency ranges is calculated.

This enables calculation of the overlapping ellipticity changingfrequency range (c) between the ellipticity changing frequency ranges(a) and (b) of the respective vibrators illustrated in the firstexemplary embodiment.

Then, in STEP 16, the vibrators 8 a and 8 b can be incorporated into thevibration-type actuator in which two vibrators relatively drive onedriving member. The vibration-type actuator can then carry outcontrollable driving as in the case of the first exemplary embodimentbased on the value calculated in STEP 15.

In the present embodiment, when a plurality of vibrators relativelydrive one driving member, and a common frequency is applied to theplurality of vibrators, each of the vibrators is mounted in the outputcharacteristics detector, which thus detects the ellipticity changingfrequency range.

Thus, the ellipticity changing frequency range (c) can be set based onthe overlapping portion between the detected ellipticity changingfrequency ranges.

Hence, when a plurality of vibrators relatively drive one drivingmember, pre-detection of the ellipticity changing frequency range ofeach of the vibrators allows easy setting of the ellipticity changingfrequency range required when the plurality of vibrators relativelydrive the one driving member.

Third Exemplary Embodiment

With reference to FIG. 7, a third exemplary embodiment will be describedwhich corresponds to an example of a configuration in which threevibrators rotationally drive a ring-shaped driving member.

In the first and second exemplary embodiments, the vibration-typeactuator in which two vibrators relatively drive one driving member hasbeen described by way of example.

In the present exemplary embodiment, as illustrated in FIG. 7, threevibrators can rotationally drive a ring-shaped driving member.

The ring-shaped driving member can perform no operations other than theone of being rotated by a guide (not shown in the drawings). In thisconfiguration, when the vibrators drive the driving member within theoverlapping potion between the ellipticity changing frequency ranges ofthe respective vibrators, the driving can be achieved with the drivingspeed prevented from being unstable.

In the present embodiment, vibrators 8 c, 8 d, and 8 e relatively driveone driving member 2, and a common frequency is applied to the vibrators8 c, 8 d, and 8 e, as illustrated in FIG. 7. When the vibrators drivethe driving member within the overlapping ellipticity changing frequencyrange between the ellipticity changing frequency ranges of therespective vibrators 8 c, 8 d, and 8 e, the driving is achieved with thedriving speed prevented from being unstable.

Furthermore, the upper limit value of the ellipticity changing frequencyrange corresponding to the highest frequency and the lower limit valueof the ellipticity changing frequency range corresponding to the lowestfrequency can be set in a manner similar to that in the second exemplaryembodiment.

Specifically, the characteristics of the respective vibrators aredetected by different characteristics detectors.

Then, based on results for the characteristics of each of the vibrators,the upper limit value of the ellipticity changing frequency rangecorresponding to the highest frequency and the lower limit value of theellipticity changing frequency range are set for the frequency rangeused for driving by the vibration-type actuator in which the vibrators 8c, 8 d, and 8 e drive the one driving member 2.

Thus, when a plurality of vibrators relatively drive one driving member,pre-detection of the ellipticity changing frequency range of each of thevibrators allows easy setting of the ellipticity changing frequencyrange required when the plurality of vibrators relatively drive the onedriving member.

As described above, the configuration according to each embodiment ofthe present invention allows stable driving to be achieved when aplurality of vibrators drives one driving member and when a commonfrequency is applied to the plurality of vibrators.

That is, even with a variation in resonant frequency between thevibrators, stable driving can be achieved in the overlapping portionbetween the ellipticity changing frequency ranges of the respectivevibrators.

Furthermore, stable driving can be achieved by adjusting the ellipticitychanging frequency range based on the output characteristics of eachvibrator.

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

This application claims the benefit of Japanese Patent Application No.2010-130156, filed Jun. 7, 2010, which is hereby incorporated byreference herein in its entirety.

1. A control apparatus for a vibration-type actuator, wherein thevibration-type actuator includes a plurality of vibrators each having acontact portion contacting an object to be driven, such that, responsiveto a common alternating current signal to the plurality of vibrators,the plurality of vibrators move, through the contact portions, theobject relative to the vibrators, to generate an elliptical motion ofthe object, wherein the control apparatus comprises a frequencydetermining unit for setting a frequency of the alternating currentsignal, and the frequency determining unit sets the frequency of thealternating current signal for changing an ellipticity of the ellipticalmotion, within an overlapping frequency range in which frequency rangesof ellipticity changing each set for each of the plurality of vibratorsare overlapped, and the frequency ranges of changing the ellipticity areset for the vibrators as a frequency range between an upper limit and alower limit, such that the lower limit is a maximum resonant frequencyin all case of changing the ellipticity, and the upper limit is largerthan the lower limit and is a maximum frequency for the relativemovement of the object.
 2. The control apparatus according to claim 1,wherein the alternating current signal is a two-phase alternatingcurrent signal, and the control apparatus further comprises anellipticity determining unit for determining the ellipticity, and fordetermining a phase difference between the two phases of the two-phasealternating current signal based on the ellipticity determined.
 3. Thecontrol method for a vibration-type actuator, wherein the vibration-typeactuator includes a plurality of vibrators each having a contact portioncontacting an object to be driven, such that, responsive to a commonalternating current signal to the plurality of vibrators, the pluralityof vibrators move, through the contact portions, the object relative tothe vibrators, to generate an elliptical motion of the object, whereinthe control method comprises a frequency determining step for setting afrequency of the alternating current signal, and, in the frequencydetermining step, the frequency of the alternating current signal is setfor changing an ellipticity of the elliptical motion, within anoverlapping frequency range in which frequency ranges of ellipticitychanging each set for each of the plurality of vibrators are overlapped,and the frequency ranges of changing the ellipticity are set for thevibrators as a frequency range between an upper limit and a lower limit,such that the lower limit is a maximum resonant frequency in all case ofchanging the ellipticity, and the upper limit is larger than the lowerlimit and is a maximum frequency for the relative movement of theobject.
 4. The control method according to claim 3, further comprising:a step for detecting the upper and lower limits, and for calculating thefrequency ranges of ellipticity changing each to be set for each of thevibrators, based on the upper and lower limits detected.